1. Field of the Disclosure
The disclosure relates generally to mesh reflectors for antennas, and more particularly relates to mesh reflectors for antennas that may be used on spacecraft, and that are adapted to be stowed in a launch vehicle and subsequently deployed in outer space.
2. Background Description
Over the past four decades, several styles of deployable mesh reflectors have been developed. The great majority of them were intended to approximate parabolic reflector surfaces, although any of them can theoretically be made to approximate other slowly varying surfaces, provided those surfaces do not have regions of negative curvature (i.e., are always curved towards the focus of the reflector). In more recent years, “shaped reflector” technology was developed and is gaining dominance in the space antenna field. So far, however, it has been limited to relatively small solid-surface (or segmented surface) reflectors due to limitations imposed by the fairing sizes of the launch vehicles on which they are flown.
Since the performance of a satellite antenna farm improves as it comprises a larger number of larger diameter reflectors, and since deployable mesh reflectors can be more efficiently packaged on a spacecraft, a greatly improved antenna farm can be produced if a deployable mesh reflector can be made to approximate an optimally-shaped reflector surface (without the “no negative curvature” limitation).
A soft knitted mesh fabricated out of a thin metallic wire (e.g., gold-plated molybdenum wire) is commonly used to form the reflective surface of deployable radio-frequency (RF) antenna reflectors, especially for space-based applications (e.g., for communication satellites). The mesh may be placed and maintained in a desired shape by attaching it to a significantly stiffer net. One problem associated with the fabrication of such a mesh surface entails the ability to maintain the tension in the mesh within a certain desired range, and to terminate/cut the mesh edges in a manner that does not produce objectionable passive inter-modulation (PIM) or electro-static discharge (ESD), through the use of an appropriate mesh edge treatment.
The problem of attaching a mesh surface to a deployable reflector's net structure entails the ability to maintain the tension distribution within the mesh as uniformly as possible as it is attached to the net, to maintain the mesh edge treatment under proper tension and wrinkle-free as it is attached to the outer catenaries of the reflector's net structure, and to minimize the effect of attaching the mesh upon the shape and the tension levels within the net structure.
The ASTRO-MESH Iso-Grid Faceted Mesh Reflector (hereinafter a “Type 1” reflector) is one example of a mesh reflector (see, e.g., U.S. Pat. No. 5,680,145). In this type of reflector, the mesh surface comprises a large number of triangular substantially flat facets. When viewed from a certain direction, the great majority of those triangles appear to be equilateral. The mesh facets are given their shape by being pulled behind a relatively stiff (ideally in extensible) set of highly tensioned straps forming a net with triangular openings. The net is pulled into shape by a set of springs pulling it backwards towards a similar (but possibly shallower) net disposed behind the mesh and curved in the opposite direction.
Another type of reflector is the Radial/Circumferential Faceted Mesh reflector (hereinafter a “Type 2” reflector). The most common examples of this type of reflector are the umbrella-style Radial-rib reflectors used on the TRW TDRS antenna, and the folding-rib reflectors currently produced by Harris Corp.
Yet another Type 2 reflector is shown and described in U.S. patent application Ser. No. 10/707,032, filed on Nov. 17, 2003, the entirety of which is hereby incorporated by reference herein. In this type of reflector, the mesh facets are generally of trapezoidal shapes bounded by a set of radial chords typically coincident with or near the location of, the reflector ribs, and by sets of chords forming concentric polygons extending between those ribs. Often, those substantially circumferential chords are made to more closely conform to the desired surface geometry by pulling down on them (i.e., in a direction pulling the surface away from the reflector focal point) with a set of adjustable tension ties. The loads in these tension ties are typically reacted by another set of chords forming a second set of concentric polygons disposed behind the set of polygons bounding the mesh facets.
Another type of reflector is known as a wrap-rib Parabolic-Cylindrically Faceted Mesh reflector (hereinafter a “Type 3” reflector). The Lockheed wrap-rib reflector has a mesh surface which comprises a relatively small number of facets each approximating a parabolic cylinder. Each of these facets is bounded by two curved parabolic ribs, an outer catenary member, and a part of the circumference of a central hub. The mesh used on these reflectors is designed to have very low shear stiffness and Poisson's ratio, which minimizes its tendency to “pillow” (or curve inwardly—i.e. towards the reflector focus—between the ribs). Typically, this type of reflector would only contain between one and several dozen facets.
“Pillowing” of a mesh is a distortion characterized by bulges (or “pillows”) that occur in the mesh due to mechanical strain. “Pillowing” in a knitted wire mesh used as a radio-frequency reflective surface generally degrades performance, and increases the levels of the side lobes of radio-frequency energy reflected from the mesh.
For acceptable RF performance (low insertion loss and low passive intermodulation (PIM)), the mesh should be kept under a certain minimum tension under all temperature conditions. For the surface “pillowing” error to be within acceptable limits, the ratio of the mesh tension to the net tension should not exceed a certain low value. The maximum net tension is limited by the available torque and force provided by the deployable reflector structure and by the desired deployment torque safety margin.
For a planar mesh to be formed into a doubly-curved surface shape, a certain variable strain should be imposed upon the mesh. The stiffer the mesh, the higher the resulting mesh strain variability.
A mesh edge treatment should be provided which will maintain the minimum required tension in the mesh all the way to the outer edge of the reflecting surface.
Upon trimming the mesh to shape, the edge treatment should restrain the cut edges of the mesh wires preventing them from unraveling and minimizing the chances of them casually contacting each other (thus causing PIM). The edge treatment should shield the cut edges of the mesh wires from viewing the antenna feed horn. The edge treatment should be kept wrinkle-free and under tension upon attaching it to the reflector net and its catenaries. The tension in the mesh should be kept as uniform as possible upon attaching it to the net. The shape of the net and its catenaries, and the tension levels in them, should not change significantly upon attaching the mesh to the net.
In prior art, mesh fabricating systems typically use rigid or semi-rigid edge strips along the outer edges (catenaries) of the mesh, and often along the gore seams to lock-in tension in the mesh from the time the mesh is laid out until it is installed on a deployable reflector structure. Systems for retention of the mesh typically use flat strips tensioned by metallic springs located behind the mesh.
Methods have been developed for making, tensioning and retaining mesh surfaces for large deployable reflectors (see, e.g., U.S. Pat. Nos. 5,969,695, 6,214,144 and 6,384,800). The mesh may be fabricated from gores which are directly sewn together and have sewn pockets at their outer edges through which outer catenary chords are passed and used to radially tension the mesh. The mesh may be given its curved shape by retaining it behind the net (i.e., on the side of the net disposed away from the reflector focus) with the members attaching the net to the reflector ribs passing through the mesh openings. No additional attachments between the mesh and the net, or mesh edge treatment, are used according to these methods.
One disadvantage of the aforementioned methods is that they can be used with a gold-plated molybdenum mesh only in non-PIM sensitive applications. In PIM sensitive applications, however, such methods are intended for use with meshes made of a material having an inherently low PIM saturation level, such as ARACON.™. fiber (material available from DuPont, fabricated out of nickel-plated Kevlar fibers). The disadvantage of using ARACON.™. fiber rather than Gold-plated Molybdenum is its increased insertion loss.
Disadvantages associated with other methods that utilize rigid or semi-rigid strips are the increased mass and stiffness associated with the use of those strips. Increased mass is undesirable particularly for space applications due the high cost associated with boosting the antenna into orbit and supporting it during the boost phase of the mission. The high stiffness of the strips is undesirable because: (1) more force is required to shape the strips into an arbitrarily shaped surface; (2) attachment of the mesh edge treatments to the net can significantly alter its tension levels and shape; and (3) it is difficult to maintain uniform tension in the strips unless additional provisions (such as tensioning springs) are added; further increasing the mass, cost, and complexity of the antenna.
While the wrap-rib type reflector can theoretically approximate a shaped surface of either positive or negative curvatures, its use for a shaped reflector application imposes other practical difficulties. Specifically, since the surface shape is provided directly by the rib shapes, it would require that each of the curved ribs be shaped differently—thus substantially increasing the cost of producing the reflector. Additionally, in order to provide enough degrees of freedom to obtain good performance, the number of ribs has to be sufficiently large to provide adequate shaping in the circumferential direction (since there are no features provided in the spans between the ribs for shaping the surface). This can result in further cost increase in addition to corresponding mass and stowed volume increases, all of which are highly undesirable.
With a Type 1 reflector, since three chords (or straps) intersect at each net node, loads can be exchanged between the chords at each node, and thus the tension can vary substantially along any one chord.
Likewise, with a Type 2 reflector, it can be shown from equilibrium analysis that the tension in the radial chords does not stay constant along the length of each chord. For example, tension in a radial chord increases substantially between the chord segments near the center of the reflector and those near its rim. As a result, if the tension at the center was at the required minimum level for an acceptable pillowing error, the tension near the outer rim of the reflector may be several times higher than that required minimum. Additionally, the tension in the circumferential members can vary as they go through each intersection, necessitating individual measurements and adjustments for each segment of each circumferential chord.
In order to guarantee the minimum tension for the life of the typical mesh reflector (and at all temperature conditions) either a substantially higher tension has to be provided to start with (as is the case with Type 1 Reflectors) or a source of flexibility (e.g., a flexible member or a spring) has to be provided to each segment.
Accordingly, there is a need for systems and methods of fabricating a reflective surface for a deployable RF antenna reflector out of a soft metallic wire mesh. Such a system should provide a means for maintaining the tension in the mesh within a certain desired range and to terminate/cut the mesh edges in a manner that does not produce objectionable PIM or ESD through the use of an appropriate mesh edge treatment.
There is also a need for systems and methods of attaching a reflective surface to a relatively stiff net defining the shape of the curved forward surface of a deployable reflector. Such a system should maximize uniformity of the mesh tension during installation, maintain the mesh edge treatment wrinkle-free, and minimize the effect of attaching the mesh upon the shape and the tension levels in the reflector net.
The present disclosure is directed to overcoming one or more of the problems or disadvantages associated with the prior art.